Post-ischemic reperfused myocardium has impaired pyruvate oxidation, accelerated fatty acid oxidation, and decreased contractile work and mechanical efficiency. Pyruvate is oxidized by pyruvate dehydrogenase (PDH) which is strongly inhibited by its product, acetyl CoA and NADH. Acetyl CoA and NADH are also the product of beta-oxidation of fatty acids, thus fatty acid oxidation inhibits pyruvate oxidation. Pharmacological stimulation of PDH or inhibition of fatty acid oxidation improves contractile recovery from myocardial ischemia. The rate of fatty acid oxidation in cardiomyocytes is primarily controlled by the activity of carnitine palmitoyl transferase I (CPT I). CPT-I is inhibited by malonyl CoA, which is synthesized in the cytosol by acetyl CoA carboxylase (ACC). Reperfused isolated rat hearts have low ACC activity and malonyl CoA, suggesting that this mechanism is responsible for the high rate of fatty acid oxidation and inhibition of carbohydrate oxidation after ischemia. It was recently demonstrated in isolated rat hearts that ACC activity is increased 2-fold by the nitric oxide second messenger cGMP. This suggests that elevated No levels would result in an increase in malonyl-CoA concentration, inhibition of CPT-1 activity, a fall in the rate of fatty acid oxidation, and a reciprocal increase in flux through PDH. Thus elevated No production during post-ischemic reperfusion might be clinically beneficial due to less fatty acid oxidation greater flux through PD, and less lactate production and hydrogen ion accumulation. The purpose of this grant is to test the general hypothesis that NO stimulation of cGMP production in the heart activates ACC and results in greater malonyl-CoA inhibition of fatty acid oxidation and reciprocal stimulation of pyruvate oxidation. Studies will be performed in the laboratory of Dr. Andrzej Beresewicz using isolated rat hearts under aerobic, ischemic and post-ischemic conditions, with measurement of glucose and fatty acid uptake and oxidation. Dr. Beresewicz has extensive experience with NO pharmacology and biochemistry, and with isolated rodent preparations, but has not worked in the field of myocardial substrate metabolism. Dr. Stanley's lab has expertise in myocardial substrate metabolism and in all of the related biochemical techniques, however he has not first hand experience with isolated perfused rat hearts with NO biochemistry. Dr. Beresewicz has extensive experience with myocardial substrate. This project will combine Dr. Beresewicz's expertise in No and the isolated rat heart with Dr. Stanley's expertise in cardiac substrate metabolism.